Picosecond light pulses are employed in a number of applications to measure the temporal response of electronic and optical materials and devices. Such pulses are conventionally produced by phase-locking the longitudinal modes of a free-running laser oscillator. Such mode-locked lasers produce a train of short pulses with a width inversely proportional to the homogeneously broadened gain linewidth of the lasing medium and a pulse-to-pulse separation equal to the round-trip time of light through the laser or to an integral sub-multiple of this time. Such mode-locked lasers are capable of producing pulses with widths as short as 0.1-10 picoseconds at pulse-to-pulse separations of the order of 1-10 nanoseconds. The length of the pulse train produced by such a laser is determined by the lasing time of the laser oscillator, which will typically vary from about a microsecond to many seconds.
Because lasing conditions may vary with time, the energy, width, and spatial distributions of the picosecond pulses produced by such lasers may evolve in time. To effectively use these short pulses for the applications referenced above, it is necessary to identify the extent of these variations over the duration of the pulse train produced by the laser.
The characterization of ultra-short (picosecond and femtosecond) laser pulses presents a number of problems. Since conventional detectors cannot be used due to their limited bandwidth, various techniques, most of them based on the measurement of the second or higher order intensity autocorrelation functions, have been proposed and tested. These techniques can be used to reconstruct, through numerical convolution, the temporal profile of the pulses. However, they typically yield only the average of the pulse shape over the wavetrain, and do not provide any information on the possible transient evolution of either the pulse shape or mode size.